Contact lenses are either of the "hard" or "soft" variety. The hard contact lenses are made from polymers with a relatively low degree of hydrophilicity and can be produced by conventional production methods for shaped objects, such as for instance, the formation of cylindrical blocks or pressing in heated moulds. In use, however, the hard lenses are often found to be unpleasant due to irritation of the eye.
The soft contact lenses are made from polymers having a high degree of hydrophilicity which, during manufacture, are swollen with water to form a gel containing at least 30% water and often over 70% water. Such lenses are soft and gelatinous, adjust extremely well to the eye and are therefore more pleasant to wear.
U.S. Pat. No. Re. 27,401 describes one class of hydrogels which can be formed from 20-97% of an aqueous liquid gelled with a sparingly cross-linked copolymer formed from a major amount of a hydrophilic monoester of acrylic or methacrylic acid cross-linked with a minor amount of a diester of these acids. The commercial soft contact lenses of this type appear to contain a polymer network formed from large amounts of hydroxyethyl methacrylate (HEMA) cross-linked with small percentages of glycol dimethacrylate. Other known hydrogels contain acrylic monomers together with vinyl pyrrolidone type polymers. See, e.g., U.S. Pat. Nos. 3,639,524 and 3,621,079. Another hydrogel described in the literature is a copolymer of acrylamide with an acrylate or methacrylate. See, e.g., U.S. Pat. No. 4,067,839.
The known methods of manufacturing soft contact lenses present a series of problems which have made the lenses costly and also have limited their use. According to one known method, the lenses are made by turning a cylindrical rod of the appropriate polymer of high hydrophilicity on a lathe. Problems are encountered because of the relative soft nature of the polymers and the high degree of accuracy required and also because of the minute dimensions of the unswollen lens. In a later process step, the lenses are swollen with water causing the lenses to change size and shape. As a result, the optical properties of each lens has to be measured after the swelling step.
Another known method of manufacture involves the casting and polymerization of suitable lens material in an open rotating mould. In this spin casting technique, the shape of the lens is determined by the centrifugal and surface tension forces acting on the resin during cure. After curing is complete, the lenses are swollen with water resulting in the same problems described above.
The more water the lenses contain, i.e., the higher the degree of swelling, the more comfortable are the lenses in use. Unfortunately, the more water used, the more the control over the final optical properties is reduced.
The mechanical strength of the lenses is also proportional to the polymer content so that higher water contents have lower mechanical strength. Moreover, lenses having higher water content are more readily attacked by microorganisms or suffer from a build up of denatured proteins and other impurities in the gel, all of which can lead to serious allergic reactions or eye disease.
The high cost of manufacture of soft contact lenses prevents systematic replacement of the lenses when contaminated and instead an elaborate system of washing the lenses overnight and frequent boiling must be established in order to maintain an acceptable hygienic state.
A new method has now been developed which permits the accurate and facile production of soft contact lenses which offer a high degree of comfort when in use coupled with a high degree of optical precision while at a price which permits replacement with fresh lenses at short intervals if so desired. These lenses have a water content of at least about 30%. At higher levels of 70-75% water, the lenses can remain in place day and night for extended periods of time up to about 14 days. At lower water levels, the lenses should be removed overnight.